The present invention relates to a rotary drive unit used in a surveying instrument and to a surveying instrument using the rotary drive unit.
A surveying instrument, e.g., a total station has a telescope unit that sights a measurement point. The telescope unit is supported by a frame unit so as to be rotatable in a vertical direction, and the frame unit is supported by a base unit so as to be rotatable in a horizontal direction. Further, the total station is provided with a motor for rotating the telescope unit and a motor for rotating the frame unit. Hereinafter, the telescope unit and the frame unit will be generically referred to as a rotation unit.
Conventionally, an adopted motor is an electric motor. An output shaft of the electric motor and a rotary shaft of the rotation unit are connected via a first gear provided to the output shaft and a second gear which is provided to the rotary shaft and meshes with the first gear. Thereby a mechanism is configured so that the rotational force of the electric motor is transmitted to the rotation unit via the first gear and the second gear. Further, a backlash exists in the mesh of the first gear and the second gear. The backlash affects a rotational accuracy and a rotational positioning accuracy, and hence the backlash must be reduced as much as possible. In particular, in surveying instruments, the accuracy of a rotation angle is required in units of seconds. Therefore, highly accurate gears are required, and further, a high accuracy is also required in an assembling accuracy. Thus, the manufacturing cost is high. Furthermore, the backlash increases due to the wear and the like of the gears, and the accuracy decreases over time.